JCL's reply is correct, but it doesn't actually answer your question. Gas heaters in touring caravans have to be what's called "Room Sealed" This means all the gas combustion takes place inside a tube which only connects to the outside air, so no gas or products of combustion will enter the interior of the caravan. The heat the flame provides is passed through the wall of the tube ( as a heat exchanger) to the air inside the caravan either directly for blown hot air or via the liquid and remote radiators in the wet systems. All you get is teh caravans internal air raised in temperature with no added moisture.
However there are several other sources of moisture in caravans. The two biggest contributors are Mammals (Humans and other animals,) who naturally breath out moisture laden gasses - an inevitable part of our breathing process.
The Institute of Specialist Surveyors and Engineers
suggests that a sedentary adult will lose about 40gms of water through breathing and perspiring every hour, and more when active.
The second major source is the cooker and oven and kettle. Gas when it burns each 1gm of gas will produce roughly 1gm of water vapour as part of its products of combustion. Both the hob, and the ovens in touring caravans vent directly into the living space and will raise the moisture levels. Kettles of course produce quite a lot of water vapour especially when they boil.
Fortunately UK and continental caravans have to be fitted with a prescribed amount of fixed ventilation. These are vents that do not have any means of closure. These should be enough to keep enough fresh air circulating to keep the caravan occupants healthy, and this air will dilute some of the water vapour produced by the above causes, but it may not be enough to prevent extreme parts of a caravan from producing some condensation.
By far the best way of reducing condensation is to increase the amount of ventilation in and out of the caravan. Raising the temperature may give the appearance of reducing humidity, becasue the ability of air to hold moisture increases with temperature, but it also needs ventilation to allow the moisture laden air to escape.
